1. Field of the Invention
The invention relates to a power supply, more particularly to a power supply including a clamp unit that has superior reliability during use.
2. Description of the Related Art
FIG. 1 illustrates a conventional power supply 900 that receives an input voltage (Vi), such as an AC voltage, from a power source 901 and outputs an output voltage (Vout) supplied to an external load 902, such as a main board. The conventional power supply 900 includes: a voltage generating unit 91 for receiving the input voltage (Vi) from the power source 901 so as to output a working voltage and a standby voltage; a monitoring unit 93 operable so as to generate a power good signal (PG) indicating whether the conventional power supply 900 outputs the output voltage (Vout), and a fault protection signal (FP) indicating whether the conventional power supply 900 is to output the output voltage (Vout); a control unit 94 for receiving the working voltage from the voltage generating unit 91 so as to output the output voltage (Vout); and an optical coupler 92 including first and second optical elements 921, 922 and coupled among the voltage generating unit 91, the monitoring unit 93 and the control unit 94.
When the conventional power supply 900 outputs the output voltage (Vout) in an ON mode, the power good signal (PG) has a high logic level, and the fault protection signal (FP) has a low logic level. Therefore, the optical coupler 92 establishes electrical connection between the voltage generating unit 91 and the monitoring unit 93, and thus electrical connection between the voltage generating unit 91 and the control unit 94 such that the control unit 94 receives the working voltage from the voltage generating unit 91 through the second optical element 922 of the optical coupler 92. On the other hand, when the conventional power supply 900 does not output the output voltage (Vout) in an OFF mode, the power good signal (PG) has a low logic level, and the fault protection signal (FP) has a high logic level. Therefore, the optical coupler 92 interrupts electrical connection between the voltage generating unit 91 and the control unit 94, i.e., the second optical element 922 does not conduct, such that the control unit 94 cannot receive the working voltage from the voltage generating unit 91.
Referring to FIG. 2, when the conventional power supply 900 is accidentally disconnected from the power source 901 during the ON mode, i.e., an input indicating signal for indicating whether the power source 901 is coupled to the conventional power supply 900 is switched from a high logic level to a low logic level, the conventional power supply 900 is switched from the ON mode to a virtual OFF mode, where the fault protection signal (FP) outputted by the monitoring unit 93 has the low logic level and where the power good signal (PG) outputted by the monitoring unit 93 has the low logic level, in response to the input voltage indicating signal. Therefore, energy attributed to the working voltage is released through the second optical element 922 of the optical coupler 92, a resistor (R2) and the external load 902 such that the output voltage (Vout) drops to zero. In this case, a warning time period (T4) is defined from the time when the power good signal (PG) is switched to the low logic level to the time when the output voltage (Vout) drops to 95% of the high logic level thereof. At the same time, energy attributed to the standby voltage is released through the first optical element 921 of the optical coupler 92, a resistor (R1) and other loads (not shown), such as a mouse and a keyboard, such that the standby voltage gradually drops. In this case, a turn off-standby time period (T5) is defined from the time when the output voltage (Vout) drops to 10% of the high logic level thereof to the time when the standby voltage drops to 95% of the high logic level thereof. It is noted that the warning time period (T4) depends on the external load 902. For example, when the external load 902 is a heavy load, the warning time period (T4) is about 4 ms, as shown in FIG. 3. When the external load 902 is a light load, the warning time period (T4) is about 53 ms, as shown in FIG. 4.
Furthermore, if the standby voltage is supplied to relatively light loads, the turn off-standby time period (T5) is prolonged. Thus, the monitoring unit 93 deactivates the control unit 94 upon detecting that the output voltage (Vout) drops to zero while the standby voltage (T5) has a high logic level. Thereafter, due to the deactivation of the control unit 94, the conventional power supply 900 cannot be reset to output the output voltage (Vout). Therefore, the conventional power supply 900 has an inferior reliability during use.